Optical device and a method of making an optical device

ABSTRACT

An optical device such as a radiation detector or an optically activated memory includes a barrier region located between two active regions. One or more quantum dots are provided such that a change in the charging state of the quantum dot or dots affects the flow of current through the barrier region. The charging states of the quantum dot is changed by an optical device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of optical devices such asphoton detectors and optical memory structures. More specifically, thepresent invention relates to photon detectors which may be configured todetect single photons and also on optically programmable non-volatilememory.

2. Discussion of the Background

There is a need for an optical detector which is capable of detecting asingle photon. Recently, this need has been heightened by the advent ofquantum cryptography of optical signals. In essence, quantumcryptography relies upon the transmission of data bits as singleparticles, in this case, photons, which are indivisible. One way inwhich the data can be encoded is via the polarisation of the electricfield vector of the photons. The key component of such a system is adetector which can respond to individual photons. It has been proposedthat quantum cryptography can be used to transmit the key for theencryption of data.

Single photon detection is also useful as a low level light detectionmeans for spectroscopy, medical imaging or astronomy. An optimum signalto noise ratio is achieved when a single photon is detected, as thenoise is then limited by the shot noise and is independent of noisearising due to the detector amplifier.

A single photon detector could also be used for time-of-flight rangingexperiments where the distance-of an object from a fixed point ismeasured by calculating the time over which a single photon takes toreturn to a detector. This technique can also be used to scan thesurface of an object, even a distant object, to form a spatial image ofits surface depth, thickness etc.

Single photon detectors are available in the form of photo multipliertubes (PMT) and single photon avalanche photo diodes (SPAD). PMTs havethe disadvantage of having low quantum efficiency, being expensive,bulky, mechanically fragile, requiring high biasing voltages andcooling. They can also be damaged and can require a long settling timeafter exposure to high light levels or stray magnetic fields. On theother hand, SPADs have the disadvantage of having a relatively low gainand high dark count rates, especially when operated at higher repetitionrates. They are also expensive and require high bias voltages andexternal cooling.

SUMMARY OF THE INVENTION

The present invention addresses the above problems of SPADs and PMTs andcan operate using low voltages (less than 5V). It is also suitable forfabrication into a multi-channel array of detectors, which is useful forspatial imaging and spectroscopy. It also has a fast time response andless noise, thus it can be used to measure the time of the arrival ofthe photon.

The detector of the present invention is completely different to thosedescribed above and uses optically stored charge to affect thetunnelling characteristics of a device.

Previously, Nakano et al, in Jpn. J Appl Physics, 36 p4283 to 4288(1997) have designed a purely electrical device to demonstrate thatcharged trapped in a layer of quantum dots can have on the resonanttunnelling characteristics of a device. This study used biases asopposed to light to cause a change in the charging state of the quantumdots. The device worked at 300K.

In a first aspect, the present invention provides an optical devicecomprising: a barrier region provided between first and second activeregions, wherein the tunnelling of carriers can occur from the firstregion to the second region through the barrier region, a first quantumdot being provided such that a change in the charging state of thequantum dot causes a change in the conditions for tunnelling through thebarrier region, and means to change the charging state of said firstquantum dot in response to irradiation.

Either or both of the first or second active regions can be bulkregions, where the carriers in those regions occupy to a continuum ofenergy levels. However, the device may also be a “low dimensional”structure where the energy of the carriers is quantised such that thecarrier can only occupy one or more discreet energy levels. Confinementof the carriers in one or more dimensions modifies this energy spectrumby a quantisation of the k-vector along the confinement direction(s). Ina quantum dot, the motion of the carriers is restricted in all threespatial dimensions. Consequently, the energy spectrum of the dotsconsists of a series of discrete levels. As the size of the quantum dotreduces, the energy spacing between these discrete levels increases. Themaximum number of electrons which can occupy each electron level is two,corresponding to the up and down spin states. Similarly, each hole levelhas an occupancy of two.

In a preferred device, the first active region is configured to allowthe flow of carriers therethrough at a first energy level, the secondlayer is configured to allow the flow of carriers therethrough at asecond energy level, and a change in the charging state of the quantumdot causes a change in the relative energies of the first and secondenergy levels.

The change in the relative energies of the energy levels affects thetunnelling conditions. However, the tunnelling current through thebarrier region will be affected to a greater or lesser degree dependingon the actual configuration of the device. Preferably, the device isconfigured such that the first and second energy levels are capable ofbeing aligned. When the first and second energy levels are aligned,resonant tunnelling can occur through the barrier layer. Under such aresonant tunnelling condition, a relatively large tunnel current willflow compared to the non-resonant case. The device is thereforepreferably configured such that a change in the charging state of thequantum dot causes-the device to be switched from being off resonance(i.e. levels not aligned) or “on resonance” (i.e. levels aligned) orvice versa.

Generally, the device will take the form of a resonant tunnelling diodecomprising an emitter and collector located on either side of thebarrier region. The detector can be configured such that carriers flowfrom the emitter to the collector when the first and second energylevels are aligned.

Therefore, a relatively large change in the tunnel current can beobserved due to the absorption of a single photon. Of course, the deviceis not limited to just detecting single photons and can also be used todetect a plurality of photons.

The device can be configured as a photon detector or an opticallyactivated memory. Preferably, for a photon detector, the device isconfigured such that the layers are not aligned i.e. the device is“off-resonance” prior to illumination.

In this arrangement, the energy levels are aligned upon illumination.Under certain operating conditions, it is believed that the flow ofcharge due to resonant tunnelling (i.e. when the levels are aligned inenergy) will re-set the charge in the dot and return the device to its“off-resonance” state. Thus, the detector resets itself.

In the resonant tunnelling diode configuration which comprises acollector and an emitter, a bias is applied across the collector andemitter in order to cause charge to flow from the emitter to thecollector. This bias will also affect the alignment of the first andsecond energy levels. Therefore, the relative energies of the first andsecond energy levels can be controlled by the application of anappropriate emitter-collector bias.

The collector and emitter may comprise metal layers. Alternatively, atleast one of the collector and/or emitter may comprise a heavily dopedsemiconductor layer. The collector and emitter may have the samepolarity or may have opposing polarities. The collector and/or emittermay form part of the first and/or second active regions. Preferably, atleast one of the collector and emitter is transparent to radiation suchthat radiation can enter the device through the collector and/oremitter.

The electron-hole pair which is generated due to incident radiation canbe generated in an absorption region of the device. In thisconfiguration, means are required to separate the electron-hole pairgenerated in the absorption region and to sweep at least one of saidelectron or hole into the quantum dot.

In an alternative configuration, the device can be configured such thatradiation excites carriers in the first quantum dot. Either interbandtransitions i.e. where an electron-hole pair is generated or intrabandtransitions where an electron or hole is excited to a further energylevel can be excited. Means can be provided to sweep a photo-excitedcarrier or photo-excited carriers out of the first quantum dot to changeits charging state.

In this configuration, radiation of only certain wavelengths (dependenton the characteristics of the device) will be able to change thecharging state of the dot. Intraband transitions which can be excitedwithin the dot often occur in the mid or far infrared part of thespectrum.

The means for sweeping carriers either to or from the first quantum dotcan comprise a means to apply a DC bias across the device, for example,electrodes may be provided on either side of the quantum dot.Alternatively, these means may comprise a Schottky gate. Where a DC biasis applied across the device in order to separate the electron-holepair, these electrodes may be the same as the emitter and collector usedto create current flow through the device. Preferably, these means aretransparent to the incident radiation.

The first and second active regions may be single layers, parts ofsingle layers or a plurality of layers. There is no requirement for thefirst and second active layers to be the same. Also, it should be notedthat the first quantum dot may lie directly in line with the tunnelcurrent or it may lie to the side of the region through which the tunnelcurrent passes.

At least one, or preferably both of the first and second active regionsare configured to be able to support low dimensional confinement ofcarriers in order to allow tunnelling therethrough. Preferably, thefirst active region will be capable of supporting two dimensionalcarrier confinement on one side of the tunnel barrier. Alternatively,the first active region may comprise a quantum dot, such thatconfinement caused by the quantum dot provides the first energy level.

The second active region may be configured to support a two dimensionalconfinement region or even zero-dimensional confinement (a quantum dot)to supply the second energy level. In order to form the second activeregion, a second barrier region is preferably provided on the opposingside of the second active region to the first barrier region.

The second barrier region is preferably configured such that alignmentof the two energy levels will cause resonant tunnelling through both ofthe barrier regions.

Although the previous discussion has concentrated on a resonanttunnelling device, any change in the tunnelling conditions, e.g. achange in the alignment of the first and second energy levels willresult in a change in the tunnel current which can be measured. However,the resonant tunnelling device is the preferred device as a large changein the tunnel current as the device is switched from on-resonance oroff-resonance will be seen.

As previously described, the detector may comprise an absorption regionconfigured to absorb incident radiation. Preferably, this region isconfigured to absorb at least 50% of the incident radiation, morepreferably at least 70%, even more preferably at least 90% of theincident radiation.

The rate of absorption absorbed by an absorption region of thickness Lis given by the formula R=R_(o)[1—exp(−α L)], where R_(o) is the rate ofincident photons and α is the absorption coefficient. The absorptioncoefficient α depends upon the material and the wavelength of theincident radiation. For example, for GaAs having a thickness of 800 nm,α=1.5×10⁶ m⁻¹. Thus, a 2 nm layer would absorb 0.3% of the incidentlights and a 1 μm thick GaAs layer will absorb 78% of the incidentlight. The absorption region may be provided on an opposing side of thesecond active layer to the first active layer.

Preferably, the device is configured so that the quantum dot traps ahole. However, it will be appreciated by those skilled in the art thatthe device would also work if the quantum dot was configured to trap aphoto-excited electron. The trapped photo electric charge trapped in thedot can have the same polarity or a different polarity to that of thetunnel current.

The quantum dot must be located so that charging of the quantum dot canaffect the relative separation of the two energy levels. The quantum dotmay be provided on either side of or anywhere surrounding the firstregion, tunnel region, second region arrangement or within any of theseregions. Preferably, the first quantum dot is formed within theabsorption layer. Alternatively, the quantum dot layer may be providedin either of the first and/or second barrier regions or within either ofthe active layers.

It has been previously mentioned that either of the first and secondactive regions could comprise quantum dots to define either or both ofthe first and second energy levels such that tunnelling occurs through aquantum dot in either or both of the first and second active regions. Inthis arrangement, either the quantum dot used to the first energy level,the ‘first active dot’ or the quantum dot used to define the secondenergy level, the ‘second active dot’, can also function to store chargein the same manner as the first quantum dot as well as allowingtunnelling. Therefore, in this arrangement, the first quantum dot iscombined with one of the active regions.

Preferably a plurality of first quantum dots is provided. A plurality oflayers or quantum dots, where each layer comprises a plurality of firstquantum dots may also be provided. The plurality of first quantum dotsmay be of different sizes. When the dots are of different sizes orcompositions, they can absorb radiation of different wavelengths. Thisis particularly useful when the device is configured such that the dotitself absorbs incident radiation.

In the preferred arrangement, the first barrier region will be a firstbarrier layer and the device will be configured in a layer by layerarrangement such that tunnelling occurs substantially perpendicular tothe plane of the barrier layer.

However, a lateral tunnelling device is also possible, where tunnellingtakes place substantially within or parallel to a plane of a layer ofthe device. Such a tunnelling device could comprise a first activeregion provided by a first quantum dot, a second active region providedby a second quantum dot where the tunnelling characteristics of thefirst and second quantum dots are affected by the charging state of athird quantum dot provided in the vicinity of the first and secondquantum dots.

The present invention can be configured as a single photon detector. Inorder to efficiently detect single photons, it is preferable if theactive area of the device, i.e. the area through which tunnellingoccurs, is less than 10⁻¹⁰ m². More preferably, this active area is atmost 10⁻¹¹ m². A single photon detector preferably comprises at most1000 first quantum dots and more preferably at most 100 first quantumdots.

Preferably, an anti-reflection coating is provided on the surface of thedetector which receives the incident radiation. The detector may beprovided in a resonant cavity in order to reflect any unabsorbedincident light back into the detector.

In order for the detector to efficiently detect light, it is preferableif any electrodes provided on the surface of the device which isincident to the radiation are essentially transparent.

A particularly useful example of the device is provided by aIn_(y)Al_(1−y)As/In_(x)Ga_(1−x)As system. Preferably in this system x isabout 0.53 and y is about 0.52. This system allows the first and/orsecond active regions to be fabricated from InGaAs and a barrier region,comprising a InAlAs layer provided adjacent the first active layer. Thebarrier region is preferably the first barrier region. The largeconduction band discontinuity between InGaAs and InAlAs potentiallyallows the device to operate at high temperatures. The conduction banddiscontinuity where x=0.53 and y=0.52 has been measured between 500 and550 meV.

Quantum dots which are formed as first quantum dots to store charge, ordots which are used to define the first and/or second energy levels arepreferably formed by depositing InAs or InGaAs.

An InGaAs absorption layer has a lower band gap than GaAs. Therefore,this absorption layer is able to absorb radiation further into theinfrared region. For example, In_(0.53)Ga_(0.47)As can absorb at theimportant wavelengths of 1.3 and 1.55 microns. These wavelengths arecommonly used for fibre optic communication. In such systems, the dotswill typically comprise InAs. Any Indium containing absorption layer isadvantageous for example, the absorption layer could be formed fromInGaAs, InGaAlAs, InGaAsSb. Another possibility is to form theabsorption layer from GaAsN.

The above system can be lattice matched to an InP substrate. This willallow the growth of a In_(0.53)Ga_(0.47)As/In_(0.52)Al_(0.48)Asstructure without lattice strain.

However, it is not mandatory to use a lattice matched substrate. Forexample, it is possible to form the above system on a GaAs substrate orany other substrate for that matter if means are provided for latticematching the lattice constant of at least one of the active regions withthat of the substrate. Such means may comprise a compositionally gradedbuffer wherein the composition of the buffer is graded such that thelattice constant of the buffer matches that of the substrate at itslower interface and that of the first active layer at its upper surface.Such a compositionally graded buffer may comprise In_(w)Ga_(1−w)As whereW changes from W=0 to W=X=0.52 gradually throughout the buffer layer.This gradual change gradually alters the lattice constant from that ofthe substrate to that of the second active layer. Other compositionallygraded buffer layers could be used, for example, AlGaAs_(x)Sb_(1−x)where x is varied.

The use of a lattice matching means such as a compositionally gradedbuffer allows a free choice of the indium content in the first activeregion. This is because the lattice constant of the compositionallygraded buffer can be tuned to any desired value.

It is also possible to use a strain relaxed buffer layer, for example aquartenary such as AlGaAsSb. Again, the strain relaxed buffer layer canbe used to provide any lattice constant for subsequent growth. Thecomposition of the quartenary can be varied in order to match thelattice constant of the first active layer. However, it should be notedthat the composition of the quartenary does not change in the same wayas described for the compositional graded buffer layer. The strainrelaxed buffer layer can accommodate dislocations.

The width of such a buffer layer is typically at least 1 μm. It is alsopossible to fabricate the device from a Si/Si_(1−x)Ge_(x) system,preferably x is about 0.3.

In a second aspect, the present invention provides an optical devicecomprising: a barrier region provided between first and second activeregion wherein tunnelling of carriers can occur from the first region tothe second region through the barrier region, a first quantum dot beingprovided such that a change in the charging state of the quantum dotcauses a change in the conditions for tunnelling through the barrierregion, and means to change the charging state of said first quantum dotin response to irradiation.

The optically activated means will preferably be provided by a quantumdot as previously described.

Preferably, in both detector in accordance with the first and secondaspects of the present invention, the quantum dots are formed by a selfassembling technique such as the Stranskii Krastanow method.

Therefore, in a third aspect, the present invention provides a method offabricating an optical device comprising: a barrier regionprovided-between first and second active regions, such that tunnellingof carriers can occur from the first region to the second region throughthe barrier region, a first quantum dot being provided such that achange in the charging state of the quantum dot causes a change in thetunnelling conditions through the barrier region, and means to changethe charging state of said first quantum dot in response to irradiation;wherein the said first quantum dot is formed as part of a layercomprising a plurality of quantum dots; the method comprising the stepsof: forming at most 10 monolayers of a quantum dot layer overlying andin contact with a layer having a substantially different latticeconstant to that of the quantum dot layer such that the quantum dotlayer forms islands, and forming a layer overlying said quantum dotlayer having a substantially different lattice constant to that of thequantum dot layer, such that a plurality of encapsulated quantum dotsare formed.

The quantum dot layer may be formed in the absorption region. Therefore,the device will preferably be formed by forming a first active region, abarrier region overlying and in contact with first active region, asecond active region overlying and in contact with said barrier region,a second barrier region overlying and in contact with said second activeregion and part of the absorption region overlying and in contact withsaid second active region. The quantum dot layer would then be formedoverlying this part of the absorption region and then the remainder ofthe absorption region would be formed on top of the quantum dot layersuch that the quantum dot layer is encapsulated within the absorptionregion.

Alternatively, the quantum dot layer could be formed in the firstbarrier region. In this situation, the barrier region would be formedoverlying and in contact with the first active region. However, onlypart of the barrier region would be formed. The quantum dot layer wouldthen be formed overlying and in contact with the partial first barrierregion and the remaining part of the barrier region would be formedoverlying and in contact with the quantum dot region such that the firstbarrier region serves to encapsulate the quantum dot layer. The secondactive region would be formed overlying and in contact with the firstbarrier region, and the second barrier region would then be formedoverlying and in contact with the second active region. The absorptionregion, if required, will then be formed overlying and in contact withthe second barrier region.

The first dot layer could also be formed in the first active region, thesecond active region or the second tunnel barrier region by forming apart of the region and then forming the quantum dot layer such that thequantum dot layer will be encapsulated by the relevant region.

Preferably, the structure would be etched through the layers to form amesa. If the structure comprises an electrode such as an emitter orcollector formed overlying the above mentioned layers, just theelectrode could be etched to define the mesa.

The structure may comprise a lower electrode. Preferably, the etch doesnot extend as far as the lower electrode in order to allow contact to bemade to the lower electrode.

As the mesa is preferably relatively small, if the device is configuredto detect single photons, it is difficult to make direct electricalcontact to an electrode on the top of the mesa. Preferably, contactmetal is provided overlying the electrode and extending away from themesa in order to make electrical contact to the mesa.

Another way to address the contact problem is to define the active areaas the area of overlap between the collector and emitter. For thisexplanation, it will be assumed that the emitter is provided at thesubstrate side of the device and that the collector is provided at theend of the mesa away from the substrate. The above described regionswill be formed on the emitter. Prior to formation of the layers, theemitter is etched into a plurality of isolated strips. The layers arethen formed on the etched emitter and the collector is then providedoverlying the layers. The collector is then etched to form a strip orplurality of strips which cross the emitter. The area or areas ofoverlap of the emitter and collector define the active area of thedevice. As both the detector and emitter extend away from the activearea, electrical contact can easily be made to both the emitter andcollector.

The optical devices of the first and second aspects of the presentinvention can also be used to fabricate a photon detector array. Thiscomprises a plurality of elements, each element comprising an opticaldevice as previously hereinbefore described configured as a photondetector. The elements may form a one dimensional array. For example,the elements may be elongate and arranged substantially parallel to oneanother. This arrangement is useful for a grating spectrometer wherelight can be dispersed in wavelength along a direction perpendicular tothe elongate elements. In this arrangement each element will detect adifferent wavelength.

The photon detector array may be a 2D array provided with a grid ofbit-lines and word-lines where each element is addressable by applyingappropriate voltage to a word-line and/or bit-line. Preferably, thebit-lines and word-lines are configured to apply a control signal to themeans for changing the charging state of the quantum dot or theoptically activated means to vary the relative energies of the first andsecond energy levels.

The array may be fabricated such that the word line is configured toapply a potential to the collector and the bit line is configured toapply a potential to the emitter or vice versa.

In a preferable configuration for a two dimensional photon detectorarray, the array will be configured such that elements in the desireddetection area are in an active state. An active state means that thedevice is configured such that upon illumination with radiation of anappropriate wavelength, the device is configured such that thetunnelling conditions will be changed. The plurality of bit-lines andword-lines are provided with current sensors such that a change in thetunnelling conditions (due to photo absorption) of an element will causea change in the measured current on both the word-line and bit-line towhich the element is connected. Thus, the exact element which absorbsradiation can be determined.

The present invention also lends itself to a memory device andspecifically a programmable non-volatile memory (PROM).

The memory essentially consists of a photon detector array as previouslydescribed. The photon array comprises a plurality of elements where eachelement comprises a photon detector as described above. Each photondetector can act as a memory element.

The array is preferably arranged as a grid of bit-lines and word-lineswhere each pixel is addressable by applying an appropriate voltage to aword-line and/or bit-line. Although, a one dimensional array is alsopossible.

For a write operation, the pixels which are to be written to areaddressed by applying an appropriate voltage to the bit-lines andword-lines. The voltage is applied so that upon illumination,photo-excited carriers are swept into or out of the dot to change thecharging state of the first quantum dot.

An appropriate bias is not applied to the unselected pixels.Photo-excited carriers will be excited in the non selected pixels.However, the photo-excited carriers will not be swept into or out of thefirst quantum dot and as a result, the charging state of the firstquantum dot will not change.

For example, if the first and second energy levels are aligned, then thedetector is ‘on-resonance’ and can be thought of as having a logic stateof ‘1’. If the detector is off resonance (i.e. levels misaligned), thedetector can be thought of as having a logic state of ‘0’. Whether ornot the detector or memory element is on or off resonance can then beeasily determined by measuring the current flowing through a pixel whenthe illumination is switched off.

To read the memory, an appropriate bias is applied to the selected pixelusing the bit-line and word-line, whether or not current flow isdependent on the charging state of the dot. As light is used to changethe charging state of the dot, in the absence of light, it is possibleto configure the device so that it will retain the charging state of thedot even if the power is switched off. Therefore, the device provides anon-volatile memory.

Preferably, current sensors-are provided to the word-lines and bit-linesto sense the change in the tunnelling conditions of the individualelements.

Another type of memory array can also be fabricated, where differentdots in each element are configured to photo absorb at differentwavelengths. This type of array provides an optically addressable memorystructure. Absorption at different wavelengths may be achieved byvarying the size or composition of the quantum dots within each element.Therefore, preferably at least one memory element comprises a pluralityof first quantum dots, and wherein at least one quantum dot in the saidelement is configured to absorb radiation with a different wavelength tothat of at least one other dot within the same element. Of course, eachelement in the array or a plurality of elements in the array could beconfigured to have dots which can absorb at a plurality of differentwavelengths.

In order to fabricate either the photon detector array or the memoryarray, it is preferable if the above photon detector is etched to formcolumns or a mesa. The columns may be etched down as far as the dotlayer and are arranged to isolate regions of preferably at most 1000dots, more preferably at most 100 dots. Alternatively, an electrodelayer provided on a top of said structure may be etched in order todefine isolated active areas of dots each comprising at most 1000 dots,more preferably at most 100 dots.

The bit-lines and word-lines may fabricate as part of the semiconductorstructure. Strips of emitter may be formed underlying the array suchthat lines of pixels are arranged with a common emitter. The collectormay then be etched to form lines which cross the emitter lines. Theareas where the emitter and collector cross are active areas of thedetector. Such an arrangement could be achieved by using a re-growthtechnique.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to thefollowing non-limiting embodiments in which:

FIGS. 1 a and 1 b show schematic band structures of a detector inaccordance with an embodiment of the present invention in a specificmode of operation;

FIGS. 2 a and 2 b show a schematic band structure of the device of FIG.1 operated at a different voltage;

FIGS. 3 a, 3 b and 3 c show schematic band structures of a detector inaccordance with a further embodiment of the present invention;

FIGS. 4 a, 4 b and 4 c show schematic band structure of a detector inaccordance with a yet further embodiment of the present invention;

FIGS. 5 a, 5 b and 5 c show schematic band structures of a furtherdetector in accordance with an embodiment of the present invention;

FIG. 6 a shows a schematic layer structure of a device in accordancewith an embodiment of the present invention, FIG. 6 b is a schematicshowing variations in the band gap of the corresponding structure;

FIG. 7 shows a schematic device structure in accordance with anembodiment of the present invention based on the layer structure of FIG.6;

FIG. 8 a shows a variation on the layer structure of FIG. 6 inaccordance with an embodiment of the present invention; FIG. 8 b;

FIG. 9 a shows a further variation on the layer structure of thedetector of FIG. 6 in accordance with an embodiment of the presentinvention, FIG. 9 b is a schematic showing variations in the band gap ofthe corresponding structure;

FIG. 10 a shows a yet further variation on the layer structure of FIG. 6in accordance with an embodiment of the present invention, FIG. 10 b isa schematic showing variations in the band gap of the correspondingstructure;

FIG. 11 a shows a yet further variation on the layer structure of FIG. 6in accordance with an embodiment of the present invention, FIG. 11 b isa schematic showing variations in the band gap of the correspondingstructure;

FIG. 12 a shows a yet further variation on the layer structure of FIG. 6in accordance with an embodiment of the present invention, FIG. 12 b isa schematic showing variations in the band gap of the correspondingstructure;

FIG. 13 a shows a layer structure in accordance with an embodiment ofthe present invention formed on a InP substrate, FIG. 13 b is aschematic showing variations in the band gap of the correspondingstructure;

FIG. 14 a shows a layer structure of a detector in accordance with anembodiment of the present invention formed on a GaAs substrate, FIG. 14b is a schematic showing variations in the band gap of the correspondingstructure;

FIG. 15 a shows a variation on the layer structure of FIG. 14, FIG. 15 bis a schematic showing variations in the band gap of the correspondingstructure;

FIG. 16 a shows a variation on the layer structure of FIG. 13 a, FIG. 16b is a schematic showing variations in the band gap of the correspondingstructure;

FIG. 17 a shows a further variation on the layer structure of FIG. 14 a,FIG. 17 b is a schematic showing variations in the band gap of thecorresponding structure;

FIG. 18 a shows a layer structure in accordance with an embodiment ofthe present invention formed using a SiGe heterostructure; FIG. 18 b isa schematic showing variations in the band gap of the correspondingstructure;

FIG. 19 a shows a variation on the layer structure of FIG. 18 a; FIG. 19b is a schematic showing variations in the band gap of the correspondingstructure;

FIG. 20 shows a pixelated device in accordance with an embodiment of thepresent invention;

FIG. 21 shows a further variation on a pixelated device in accordancewith an embodiment of the present invention having a common emitter;

FIG. 22 shows a schematic memory device in accordance with an embodimentof the present invention; and

FIG. 23 a shows a schematic cross-section of a memory device or detectorarray, FIG. 23 b shows the corresponding plan view.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A and FIG. 1B are schematic band structures which are used toillustrate a mode of operation for a photon detector.

A conduction band 1 and a valence band 3 is shown. FIG. 1A shows thedetector prior to illumination. FIG. 1B shows the detector afterillumination.

An emitter 5 and a collector 7 are provided at either end of thedetector. An emitter-collector bias V_(ce) is applied across thedetector such that the potential of the collector 7 is lower than thatof the emitter 5, thus inducing the flow of electrons from the collectorto the emitter. In this example, the carriers will be electrons.However, it will be appreciated by those skilled in the art thatdetector could be configured with holes as the majority carrier.

The detector comprises a first low dimensional system 9 which is locatedbetween the emitter 5 and the first barrier layer 11. Electrons in thelow dimensional layer 9 have an energy of the first energy level 13(this level can be seen more clearly on FIG. 1B). Adjacent the barrierlayer 11 and on the opposing side to the first low dimensional system 9is a second low dimensional system 15. The second low dimensional system15 is capable of confining electrons with a second energy level 17. Inthe detector shown in FIG. 1A (before illumination), the first energylevel 13 and the second energy level 17 align.

Adjacent the second low dimensional system 15 is a second barrier layer19. The second barrier layer 19 is thin enough so that when the first 13and the second 17 energy levels align, resonant tunnelling takes placethrough the first barrier layer 11 and the second barrier layer 19. Anabsorption layer 23 is then provided between the second barrier layer 19and the collector 7. A quantum dot layer 21 is then provided in saidabsorption layer 23 on the opposing side of the second barrier layer tothat of the second low dimensional system 15.

Due to the alignment between the first and second energy levels 13, 17,charge flows freely from the emitter 5 to the collector 7 when a biasV_(ce) is applied. The alignment of the first and second energy levelswill be dependent on the magnitude of the applied bias. The magnitudeV_(ce) is chosen such that the energy levels 13 and 17 align.

FIG. 1B shows the same device as that of FIG. 1A. However, the devicehas been illuminated. To avoid unnecessary repetition, like numeralshave been used to denote like features. On absorption of a photon, anelectron-hole pair is excited, here shown as electron 25 in theconduction band and hole 27 in the valence band. The bias V_(ce) causesthe electron 25 to be swept towards the collector. However, the hole 27is swept in the opposite direction and is swept into dot 21 where it istrapped. The change in the charging state of dot 21 causes a change inthe alignment of the first energy level 13 and the second energy level17. As these two levels do not now align, the detector is brought“off-resonance” and tunnelling through the first barrier layer issuppressed.

Therefore, the charge cannot flow freely from the emitter 5 to thecollector 7. This change in current can easily be detected and signifiesthe absorption of a single photon.

In the above device, the photon detector is switched on by applyingcollector/emitter bias V_(ce) across the detector. When a single photonis absorbed, the emitter/collector current is reduced.

FIG. 2 shows operation of the device of FIG. 1 at a different bias V′ce.To avoid unnecessary repetition, like reference numerals will be used todenote like features. In the case of FIG. 1, the device was on-resonancei.e. the first and second energy levels were aligned, beforeillumination. In the device of FIG. 2, the first energy level 13 and thesecond energy level 17 are misaligned prior to photon absorption. Afteran electron-hole pair has been excited as shown in FIG. 2B, the chargingof quantum dot 21 with a hole causes the first 13 and second 17 energylevels to align, thus allowing charge to flow freely from the emitter 5to the collector 7.

In this mode of operation, the detector is again switched on by applyinga bias across the collector/emitter. However, current flow is suppressedsince the first and second energy levels do not align. Uponillumination, absorption of a single photon and subsequent capture ofone of the photo-excited carriers by a quantum dot causes the levels toalign. Hence, there is a detectable increase in the current.

In both of FIGS. 1 and 2, photon is absorbed in the absorption layer.However, the device can also be configured such that incidentillumination causes absorption of a photon in the quantum dot. This typeof device is shown in FIGS. 3 to 5.

FIG. 3 a shows a variation on the device of FIGS. 1 and 2. To avoidunnecessary repetition, like reference numerals will be used to denotelike features. The device operates at emitter 5/collector 7 biasV″_(ce). This bias serves to misalign the first 13 and second 17 energylevels prior to illumination. Second barrier layer 19 is thicker in thedevice of FIG. 3 than those shown in FIGS. 1 and 2. Also, instead ofabsorption layer, buffer region 24 is provided. Buffer region 24 may beidentical to the absorption layer 23 described in relation to FIGS. 1and 2. However, in this particular example, there is no need for bufferlayer 24 to be able to absorb incident radiation. The device isilluminated with radiation having a frequency substantially equal tothat of an interband optical transition of the quantum dot 21. Thiscauses photo excitation of an electron 25 into the conduction band ofdot 21 and a hole 27 into the valence band of dot 21. This excited stateis not stable and the electron 25 tunnels out of dot 21 towards thecollector 7. This leaves just hole 27 in the valence band 3. Tunnellingof the hole 27 from the valence band towards the emitter 5 is largelysuppressed by the thick barrier layer 19.

FIG. 3 c shows a stable condition where the hole 27 is trapped in dot 21and provides a positive charge. This change in the charging state of dot21 affects the alignment of energy levels 13 and 17 such that the levelsalign. The alignment of these levels allows resonant tunnelling to occurthrough barrier layers 11 and 19.

FIG. 4 shows a variation on the device of FIG. 3. To avoid unnecessaryrepetition, like reference numerals will be used to denote likefeatures. The principle of operation of this device is very similar tothat of FIG. 3. However, in this example, an electron remains trappedwithin the quantum dot 21 and hole 27 tunnels out of the quantum dottowards emitter 5. In this specific example, the dot 21 is located inthe second active layer 15 between the first 11 and second 19 tunnelbarriers.

The device operates at an operating voltage of V′″_(ce). At this bias asshown in FIG. 4 a, the first energy level 13 and the second energy level17 are misaligned. Upon illumination with radiation having a frequencysubstantially equal to that of the band gap of the quantum dot, anelectron 25 and hole 27 are photo-excited within the quantum dot 21.

As this state is not stable, hole 27 tunnels out of the quantum dot 21into the emitter 5. This is shown in FIG. 4 b. Tunnelling of theelectron 25 towards the collector 7 is largely suppressed due to thethick barrier layer 19.

FIG. 4 c schematically illustrates a situation where just the electron25 is trapped within quantum dot 21. This negative charge on the dot 21causes alignment of first energy level 13 with second energy level 17.This will then allow resonant tunnelling carriers through barriers 11and 19 and hence, a larger current can flow from the emitter 5 to thecollector 7.

The above four figures have all shown the situation where an interbandtransition occurs, i.e. carriers are excited into the conduction orvalence bands. However, the device can also work in an intraband modewhere illumination just serves to excite carriers within the conductionband or valence band. A device working on this principle is shown inFIG. 5.

FIG. 5 a shows the device prior to absorption. The structure of thedevice is essentially identical to that of FIGS. 1 and 2. Therefore, toavoid unnecessary repetition, like reference numerals will be used todenote like features. The device is operated at an emitter 5/collector 7bias of V^(1V) _(ce). At this bias, the first energy level 13 is alignedwith the second energy level 17 so that tunnelling occurs through thebarriers 11 and 19. Therefore, in this situation, current can flowfreely from the emitter 5 to the collector 7. In this state, an electron25 is located within the conduction band 1 of dot 21.

FIG. 5 b shows the situation after illumination with radiation having anenergy substantially equal to that of an intraband transition within thequantum dot 21. Electron 25 is excited to a higher energy state fromwhich it can be tunnelled out of the quantum dot 21. After illumination,the electron 25 which is now no longer contained within quantum dot 21flows toward the collector 7. This causes a change in the charging stateof dot 21 which affects the alignment of first energy level 13 andsecond energy level 17. This is shown in FIG. 5 c. As the first andsecond energy levels are not aligned, resonant tunnelling cannot occurand charge does not flow freely from the emitter 5 to the collector 7.

FIG. 6 shows a schematic layer structure of a device in accordance withan embodiment of the present invention. An emitter layer 31 which willpreferably be a highly doped semiconductor layer is formed overlying andin contact with buffer layer 34. Buffer layer 34 is formed overlying andin contact with substrate 33. A lower spacer layer 35 is then formedoverlying and in contact with the emitter layer 31. A first barrierlayer 37 is then formed overlying and in contact with first spacer layer35. The first spacer layer 35 and first barrier layer 37 are configuredsuch that the junction between the first barrier layer 37 and the firstspacer layer 35 is capable of supporting a low dimensional system forexample, it could be capable of supporting two dimensional confinement.

A quantum well layer 39 is then formed overlying and in contact with thebarrier layer 37. The quantum well layer 39 is capable of supporting lowdimensional confinement of the carriers such as a two dimensionalconfinement gas. A second barrier layer 41 is then formed overlying andin contact with said quantum well layer 39. A second spacer layer 43 isthen formed overlying and in contact with said second barrier layer 41.A layer of quantum dots 45 is then formed overlying and in contact withsaid second spacer layer 43. A third and final spacer layer 47 is thenformed overlying and in contact with said quantum dot layer 45 and saidsecond spacer layer 43. The second 43 and third 47 spacer layerstogether form an absorption layer. A collector layer which comprises ahighly doped semiconductor layer 49 is then provided overlying and incontact with said absorption layer 47. A schematic showing the variationin the band gap of the structure of FIG. 6 a is shown in FIG. 6B. Thefeatures of FIG. 6 a have been numbered on FIG. 6 b.

FIG. 7 shows the layer structure of FIG. 6 after fabrication.

The structure is etched down to the emitter 31 to form mesa 51 such thatat least part of emitter 31 extends away from mesa 51 such that acontact 53 can be made to the emitter away from the mesa 51. Emittercontact 53 is preferably an ohmic contact such as those made fromNiGeAu. The mesa may have an area of at most 10⁻¹⁰ m². Therefore, it isdifficult to form a single isolated contact at the top of mesa 51.Instead, contact metal will be applied to the mesa which will extendfrom the collector away from the mesa, such that a collector contact canbe made away from the mesa 51. Methods for contacting the collector willbe described in more detail with reference to FIG. 23.

The operation of the device has been described with reference to FIGS. 1to 5. The first low dimensional system layer 9 is formed at the junctionof first spacer layer 35 and first barrier layer 37. The first andsecond barrier layers 11, 19 of FIGS. 1A and 1B correspond to the firstand second barriers layer 37 and 41 of FIG. 6. The second confined lowdimensional system 15 corresponds to quantum well layer 39 in FIG. 6.

In FIG. 6, a plurality of quantum dots are shown in quantum dot layer 45When operating as a single photon detector, the detector will generallyonly be required to detect one photon at a time. A single photon willresult in a carrier being trapped in a single quantum dot.

FIG. 8 shows a variation on the structure of FIG. 6. To avoidunnecessary repetition, like reference numerals will be used to denotelike features. The layer formation shown in FIG. 8 is similar to thatshown in FIG. 6 up to the formation of the first tunnel barrier 37.However, the quantum dot layer 45 which is provided between the secondand third spacers in FIG. 6 (i.e. within the absorption layer) is nowformed within the quantum well layer 39. This structure would befabricated by forming the first half of the quantum well layer 39,forming the quantum dot layer 45 and then forming the remainder of thequantum well layer 39. The second tunnel barrier 41 is then formedoverlying the quantum well layer 39. A third spacer layer 47 (whichforms an absorption layer) is then formed overlying the second barrierlayer 41. The collector 49 is formed as previously described. Thedetector is then fabricated in the same way as described with referenceto FIG. 7.

FIG. 8B schematically shows how the band gap varies for this device. Theconduction band 61 and the valence band 63 are shown. Looking at theconduction band 61, the two barrier layers 37 and 41 are shown asmaximums in the conduction band 61. The quantum well layer 39 is showninterposed between the two barrier layers 37, 41 and the quantum dotlayer 45 is shown located within the quantum well 39. Following thearrangement described with reference to FIGS. 1 and 2, uponillumination, a hole is swept from the absorption layer 47 into thequantum dot 45 which affects the relative separation of the first andsecond energy levels. This device could also be configured to operate inthe manner described with reference to FIGS. 3 to 5.

FIG. 9 shows a further variation on the basic structure of FIG. 6.Again, to avoid unnecessary repetition, like features will be denotedwith like reference numerals.

Here, the quantum dot layer 45 is provided within the second tunnelbarrier 41. The fabrication of the device remains identical to thatdescribed for FIG. 6 up to quantum well layer 39. The second tunnelbarrier 41 is formed in a two stage process where the first half of thetunnel barrier is formed, then the quantum dot layer 45 is formed, thenthe remainder of the second tunnel barrier layer 41 is formed. The thirdspacer layer 47 is then formed overlying and in contact with the secondbarrier layer 41. The collector 49 is then formed as previouslydescribed. The detector is then fabricated in the same way as describedwith reference to FIG. 7.

The variations in the band gap of this structure are shown schematicallyin FIG. 9B. Here, the quantum dot, 45 forms a minima in the secondtunnel barrier 41. As described with reference to FIGS. 1 and 2, in thearrangement shown in FIGS. 1 and 2, a photo generated hole is swept intothe second barrier layer 41 and trapped in the quantum dot 45 therebyaffecting the relative separation of the first and second energy levels.The device could also be configured to operate in the manner describedwith reference to FIGS. 3 to 5.

FIG. 10 shows a further variation on the device of FIG. 6, here, twoquantum dot layers are used. Comparing the device exactly with that ofFIG. 6, like reference numerals are used to denote like features toavoid unnecessary repetition.

The quantum dot layer 45 provided within the absorption layer 47 asdescribed with reference to FIG. 3. However, the quantum well layer 39is substituted with a second quantum dot layer 40 which will be referredto as the second active dot layer 40. This layer 40 provides the secondlow dimensional system 15 as described with reference to FIGS. 1 and 2.The second active dot layer 40 is formed in the same manner as dot layer45 in FIG. 8.

The structure is then processed as described with reference to FIG. 7.

FIG. 11 a shows a variation of the device of FIG. 10. Here, a thirdquantum dot layer is provided in the first tunnel barrier 37. The secondspacer layer 43 is removed and the first quantum dot layer 45 is formedoverlying and in contact with the second tunnel barrier layer 41. Thisthird quantum dot layer provides the first low dimensional system 13, asdescribed with reference to FIGS. 1 and 2 and will be referred to as thefirst active dot layer 38. The first active dot layer 38 is formed inthe same manner as dot layer 45 described with reference to FIG. 6.

In operation, carriers will become trapped in the first quantum dotlayer 45 therefore changing the alignment of the energy levels of thefirst 38 and second 40 active dot layers. However, it will beappreciated by those skilled in the art that if the first and secondactive layers are provided by quantum dots, then there is no need tohave a separate quantum dot to store the charge.

This arrangement is shown in FIG. 12. Here, the second tunnel barrier 41and the first quantum dot layer 45 removed. Alignment of the energylevels of the second and third quantum layers allow resonant tunnelling.However, upon illumination, a hole can be swept into either the first orsecond active dot layer which will affect the alignment of the energylevels and hence will vary the tunnelling characteristic of the device.

FIG. 13 shows a specific structure formulated using the InGaAs/AlInAssystem on an InP substrate. The preferred system usesIn_(0.53)Ga_(0.47)AS/Al_(0.48)In_(0.52)As. To avoid confusion, the samereference numerals will be used to denote the same functional layers asused with reference to FIGS. 3 to 7.

An InP substrate 33 is used. An In_(0.52)A_(0.48)As matching layer 32having a thickness of 300 nm is then formed overlying and in contactwith said InP substrate 33. The In_(0.52)Al_(0.48)As layer 32 is formedin order to initiate the growth. This layer could be doped to have thesame polarity and dopant concentration as the emitter or could beundoped. An InGaAs buffer layer 34 having a thickness of 100 nm is thenformed overlying and in contact with said matching layer 32.

An InGaAs emitter layer 31 which is doped with Si donors having aconcentration of 2×10¹⁸ cm⁻³ and has a thickness of 500 nm is thenformed overlying and in contact with said buffer layer 34.

A first spacer layer 35 of undoped InGaAs with a thickness of 50 nm isformed overlying and in contact with said heavily doped emitter layer31. A first barrier layer 37 formed from 10 nm AlInAs is formedoverlying and in contact with first spacer layer 35. A quantum welllayer 39 having a thickness of 8 nm and comprising undoped InGaAs isformed overlying and in contact with said first barrier layer 37. Asecond barrier layer 41 formed from 10 nm of undoped AlInAs is formedoverlying and in contact with said quantum well layer 39. A secondspacer layer 43 comprising InGaAs which is undoped and has a thicknessof 2 nm is formed overlying and in contact with said second tunnelbarrier 41.

The temperature of the growth is then dropped to 530° C. and between 1.6and 4 monolayers of InAs is formed overlying and in contact with saidsecond spacer layer 43. Due to the low growth temperature and thesubstantial difference in lattice constant between InAs and InGaAs, thelayer forms islands. A third spacer layer 47 of InGaAs having athickness of 1000 to 2000 nm is then formed overlying and in contactwith the second spacer layer 43 and the dot layer 45. This formsencapsulated dots. The second 43 and third 47 spacer layers togetherform an InGaAs absorption layer. A heavily doped 10¹⁸ cm⁻³ (with Sidonors) InGaAs layer of 80 nm is formed overlying and in contact withsaid third spacer layer 47. This heavily doped layer 49 forms thecollector. The collector 49 may also be made from InAlAs to avoid photoabsorption in this layer.

FIG. 13B shows the variations in the band gap of the layer structure ofFIG. 10A. To avoid unnecessary repetition, like numerals have been usedto denote like features. The band structure is schematic in that onlylayers which cause a large change in the energy of the conduction orvalence bands are shown.

FIG. 14 shows a further material system for forming a detector or memoryin accordance with an embodiment of the present invention. Thisstructure is formed on a GaAs substrate 33. Where possible, likereference numerals are used to denote functional like features with thatof FIG. 6.

A GaAs buffer layer 32 having a thickness of 500 nm is formed overlyingand in contact with said GaAs substrate 33. A 1000 to 2000 nm thickInGaAs graded buffer layer 34 is then formed in contact with andoverlying said GaAs buffer layer 32. The In composition of the gradedbuffer layer changes from 0 to x over the course of its growth.Therefore, it is lattice matched to GaAs buffer layer 32 andIn_(x)Ga_(1−x)As the following layer 31. The graded buffer layer may bedoped or undoped. This InGaAs layer forms emitter 31. This emitter andthe following layers are identical to those described with reference toFIG. 11, except the In content of the In_(x)Ga_(1−x)As andIn_(y)Al_(1−y)As may be different. A schematic diagram showing thevariation in the band gap of the layers is shown in FIG. 12B. Again,like reference numerals have been used to denote like features.

FIG. 15 a shows a yet further variation on the layer structure of FIG.13. Where possible, like reference numerals have been used to denotelike features. The device forms the same plan as that shown in FIG. 6where the dot layer is formed within the absorption layer.

The device is formed on a GaAs substrate 33. AlAs buffer layer 34 havinga thickness of 5 nm is then formed overlying and in contact with saidGaAs substrate 33. This is then followed by a AlSb layer 34 a which hasa thickness of 35 nm. These two layers help to initiate the growth.

A quaternary buffer layer 32 is then formed of AlGaSbAs. For instance,(Al_(0.5)Ga_(0.5))(As_(0.55)Sb_(0.45)) This buffer layer has a thicknessfrom 1000 to 2000 nm. The remainder of the layers are then identical tothose described with reference to FIG. 14. A diagram showing thevariation in the band gap of the layer structure of FIG. 15 a is shownin FIG. 15 b.

FIG. 16 a shows a further material system for fabricating the device inaccordance with an embodiment of the present invention. The device isformed on an InP substrate 33.

An undoped InP buffer layer 34 having a thickness from 1000 to 2000 nmis then formed overlying and in contact with said InP substrate 33. Itshould be noted that the buffer layer could also be doped like theemitter. A heavily doped 2×10¹⁸ cm⁻³ (Si donors) n type emitter 33formed from 500 nm of InP is then formed overlying and in contact withsaid buffer layer 34. A first spacer layer 35 comprising 50 nm InP isformed overlying and in contact with said emitter 31.

A first barrier layer 37 comprising 10 nm of undoped InAlP is formedoverlying and in contact with said first spacer layer 35. A quantum welllayer 39 comprising 8 nm of undoped InP is formed overlying and incontact with said first barrier layer 37. A second barrier layer 41comprising 10 nm of undoped InAlP is formed overlying and in contactwith said quantum well layer 39. A second spacer layer 43 comprising 2nm of undoped InP is formed overlying and in contact with said secondtunnel barrier layer 41. A layer of quantum dots 45 is then formedoverlying and in contact with said second spacer layer 43. These dotsare formed in the same manner as those described with reference to FIG.10. This is because there is a large difference in the lattice constantbetween InP and InAs. The remainder of the layers are identical to thosedescribed with reference to FIG. 13.

A diagram showing the variations in the band gap of the layer structureof FIG. 16 a is shown in FIG. 16 b.

FIG. 17 shows a further system which can be used to fabricate a devicein accordance with the present invention. The system uses a GaAssubstrate 33. A GaAs buffer layer 34 having a thickness from 1000 to2000 nm is formed overlying and in contact with said substrate 33. Itshould be noted that the buffer layer 34 could also be doped to have thesame polarity as that of the emitter. An emitter layer 31 comprising 500nm of n+doped (2×10¹⁸ cm³¹ ³) GaAs is formed overlying and in contactwith said buffer layer 34. A first spacer layer 35 comprising 50 nm ofundoped GaAs is formed overlying and in contact with said emitter 31. Afirst tunnel barrier layer 37 comprising 10 nm of undoped AlGaAs isformed overlying and in contact with said first spacer layer 35. Thequantum well layer 39 formed from 8 nm of undoped GaAs is formedoverlying and in contact with said first tunnel barrier layer 37. Thesecond tunnel barrier layer 41 formed from 10 nm of undoped AlGaAs isformed overlying and in contact with said quantum well layer 39.

The second spacer layer of GaAs 43 having a thickness of 2 nm is formedoverlying and in contact with said second barrier layer 41. A quantumdot layer 45 formed from 1.6 to 4 monolayers of InAs is then formed inthe same manner as described with reference to FIG. 7. Due to the largevariation in lattice constant between InAs and GaAs, the quantum dotlayer forms islands. A third spacer layer comprising from 1000 to 2000nm of GaAs is then formed overlying and in contact with said secondspacer layer and quantum dot layer 45. A collector 49 comprising 80 nmof doped (2×10¹⁸ cm⁻³ Si) GaAs is then formed overlying and in contactwith said third spacer layer.

FIG. 18 shows a further system which can be used to fabricate a devicein accordance with an embodiment of the present invention. Here, thedevice is fabricated using the SiGe/Si system. A Si buffer layer 34having a thickness of 1000 nm which is nominally undoped is formedoverlying and in contact with the Si substrate 33. An emitter layer 31which comprises 500 nm of p doped (2×10¹⁸ cm⁻³) Si is then formedoverlying and in contact with said buffer layer 34. This layer acts asthe emitter. A first spacer layer comprising 50 nm of undoped Si is thenformed overlying and in contact with said emitter layer 31. A firstquantum well layer 36 comprising 5 nm of undoped Si_(0.7)Ge_(0.3) isthen formed overlying and in contact with first spacer layer 35. A firsttunnel barrier layer 37 comprising 8 nm of Si is formed overlying and incontact with said first quantum well layer 36.

A second quantum well 39 comprising 5 nm of undoped SiGe is then formedoverlying and in contact with said first tunnel barrier layer 37. Asecond tunnel barrier layer 41 comprising 8 mm of undoped Si is thenformed overlying and in contact with said quantum well layer 39.

Several monolayers of Ge are formed overlying and in contact with thesecond tunnel barrier layer 41. Due to the substantial difference inlattice constant between Ge and Si, the layer forms islands. Thirdspacer layer 47 comprising 2000 nm of undoped Si is then formedoverlying and in contact with said dot layer 45 to encapsulate the dots.A collector layer comprising 100 nm of p doped (2×10¹⁸ cm⁻³) Si is thenformed overlying and in contact with said third spacer layer 47.

FIG. 18 b is a diagram showing schematically the differences in the bandgap of the layers of the structure shown in FIG. 18 a.

FIG. 19 shows a variation on the layer structure of FIG. 18. A gradedbuffer layer 34 comprising from 1000 to 2000 nm of Si_(0.7)Ge_(0.3) isformed overlying and in contact with said Si substrate. The buffer layeris graded so that it uniformally changes across its width in the growthdirection from having 0 Ge content at the interface with the Sisubstrate to having a Ge content of 0.3. P+ emitter 31 comprises 500 nmof doped (2×10¹⁸ cm⁻³) Si_(0.7)Ge_(0.3). A first spacer layer comprising50 nm of undoped SiGe is then formed overlying and in contact with saidemitter layer 31. A first tunnel barrier layer 37 comprising 8 nm Si isthen formed overlying and in contact with said first spacer layer 35.Due to the variations in the band gap between the first spacer layer 35and the first tunnel barrier layer 37, a quantum well is formed at theheterojunction between the first spacer layer 35 and the first tunnelbarrier 37.

A quantum well layer 39 comprising 5 nm of undoped Si_(0.7)Ge_(0.3) isthen formed overlying and in contact with said first barrier layer 37. Asecond barrier layer 41 comprising 8 nm of undoped Si is then formedoverlying and in contact with said first quantum well layer 39. A secondspacer layer comprising 5 nm of undoped SiGe is then formed overlyingand in contact with second tunnel barrier layer 41. A quantum dot layeris then formed from depositing several monolayers of Ge. A third spacerlayer 47 comprising 2000 nm of Si_(0.7)Ge_(0.3) is then formed overlyingand in contact with said quantum dot layer 45 to encapsulate the quantumdots. A collector comprising 100 nm of p doped (2×10¹⁸ cm⁻³)S_(0.7)Ge_(0.3) is then formed overlying and in contact with said thirdspacer layer.

All of the layer structures described with reference to FIGS. 13 to 19can be processed as described with reference to FIG. 7.

FIG. 20 shows a pixelated device made out of a plurality of photondetectors of the type described with reference to FIGS. 1 to 19. Thedevice shown here is similar in structure to that described withreference to FIGS. 6 and 7. However, it will be appreciated by thoseskilled in the art that the variations shown in FIGS. 8 to 12 could alsobe employed here.

The device has a mesa which is defined such that a plurality of elements20 are formed. The elements are formed on a common substrate 101 and acommon emitter layer 103. Each element has preferably an upper area ofat most 10⁻¹⁰ m² if the device is to be used as a single photondetector.

The photon detector array is formed on top of a substrate/buffer 101.The substrate/buffer can be any of those described with reference toFIGS. 6 to 19. An emitter layer 103 is then formed overlying and incontact with said substrate/buffer layer 101. The following layers canthen be the same as any those described with reference to FIGS. 6 or 8to 19. The layers are then etched down to the emitter to form aplurality of elements 105, each element being separated by a trench 107(formed by a mesa etch).

FIG. 20 shows a cross section through the array for simplicity. Thearray can be a one dimensional array, where the elements 108continuously extend perpendicular to the plane of the paper. Thisconfiguration is useful for a grating spectrometer, where light can bedispersed along a direction perpendicular to the strips so that eachstrip/element detects a different wavelength.

Alternatively, the elements can form a 2D array of pixels.

The emitter layer 103 is common to each of the elements 105. The emitterlayer also extends away from the elements to form the room for emittercontact 109.

Each photon detector element can be switched on by applying anappropriate collector/emitter bias. The bias can be chosen either toalign the first and second energy levels when the dots are uncharged orit can be chosen so that the first and second energy levels will alignwhen a quantum dot is charged.

Connecting to the collector is difficult if the elements are small. Insuch situations, contact metal will be evaporated over the etchedstructure such that electrical contact can be made to the collectorremote from the element 105.

Taking, for example, the situation where the collector/emitter biasaligns the first and second energy levels when the dots are in anuncharged state, when one of the elements 105 absorbs a single photon,current flow through that element is largely suppressed. If individualcontact is made to each element, then it is possible to determine theexact element which has absorbed the photon.

FIG. 21 shows a variation on the device of FIG. 20. Here, the mesa etchwhich defines trench 107 just extends into the absorption layer. Thisshallow etch allows separate elements to be formed. Also, as the etchcan cause defects which affect the tunnelling characteristics of thedevice, it can be advantageous to avoid etching the tunnelling region.

The photon detector can also be configured as a memory device. In awrite mode, the elements 105 which are to be written to have acollector/emitter bias applied to them which will allow a carrier to beswept into or out of the dot, as required The device can be illuminatedwith a broad beam of radiation as only the elements with the correctapplied voltage will have the charging state of the quantum dot changedi.e. those elements in an “active” state. The device can then be read byapplying an appropriate collector/emitter bias to each of the elements.The collector/emitter bias can be chosen so that elements which haveuncharged dots conduct and elements which have charged dots do not orvice versa

FIG. 22 shows a schematic device configured as a memory structure. Thememory elements M (x,y) each contain a photon detector as previouslydescribed. The structure comprises a grid of parallel word lines (WL)which are perpendicular to bit lines (BL). The word lines are in thisexample connected to the emitter, (they could be connected to thecollector) and the bit lines are connected to the collector. By applyinga specific bias on a particular word line and a bit line, a potentialdifference can be applied across the collector and emitter of a selectedmemory cell M (X,Y).

More than one memory cell can be selected if required.

During a write operation, appropriate voltages are applied on thebit-lines and word-lines to activate the selected memory cells so thatphoto-excited carriers are trapped within the dots. The device can beconfigured such that when the power to the word-lines and bit-lines isdisconnected, the dots still trap charge. This means that the memory canact as a non-volatile memory. In this situation, it is preferable if thedetector is configured such that it is on resonance prior toillumination. Thus, there is little chance of the device resettingitself when the tunnel current flows.

To read the memory, current through the bit lines and word lines aresensed using a current sensing means. The current through an elementwill change dependent on the tunnelling conditions. By measuring thecurrent passing along the word line and bit line to which a specificelement is connected, it is possible to determine the current throughthe element and hence, the charging state of the dot or dots.

A further type of memory device is also possible using the sameprinciples as above. The wavelength of radiation absorbed by a quantumdot is dependent on the size of the dot and its composition. It ispossible to fabricate a memory array where each element comprises aplurality of first quantum dots and where the first quantum dots are ofdifferent sizes such that different dots absorb light of differentwavelengths. Thus, an optically addressable as well as a spatiallyaddressable memory can be formed.

FIG. 23 a shows a cross-section of the device of FIG. 23 b. FIG. 23 ahas a similar layer structure to that of FIG. 20.

The plan view 23 b shows a plurality of parallel lines which areemitters 121. These may be formed by growing a heavily doped layer andetching the layer to form strips. The remaining layers will then beformed overlying the emitter in a manner described with reference to anyof the preceding figures. The collector will be formed as a heavilydoped layer overlying the whole structure. The collector is then etchedto form strips 123. Each pixel is defined by the overlap of emitters 121and collectors 123. The emitters can function as word-lines whereas thecollectors can function as bit-lines or vice versa.

Previously, we have discussed the use of bit-lines and word-lines toaddress each pixel where the collector or emitter is connected to theword-line and the remaining electrode is connected to the bit-line.However, it is also possible to fabricate the device by providing a lineof elements all in series with a word-line and a perpendicular line ofelements all in series with a bit-line. This will also allow a change incurrent or the current from a particular element to be monitored. Acurrent sensing circuit can be provided in series with each word-lineand each bit-line.

Although the above discussion of pixelated devices has primarilyconcentrated on memory devices, it is also possible to use such anarrangement for an array of photon detectors. An active area of a photondetector array can be defined by setting all of the elements in thatpart of the array to an “active” state via an appropriate bias on theselected word-lines and bit-lines. Current sensors are provided on saidbit-lines and word-lines. When a photon is absorbed by a single element,a current change will be sensed on the bit-line and word-line which areconnected to that element. Thus, such an array can be used to detectphotons and to determine the exact point of photon absorption.

1. An optical device comprising: a barrier region provided between firstand second active regions, and configured to allow tunnelling ofcarriers from the first region to the second region through the barrierregion; at least one first quantum dot configured such that a change inthe charging state of the first quantum dot causes a change inconditions for the tunnelling of carriers through the barrier region;and means for changing the charging state of said first quantum dot inresponse to irradiation.
 2. The optical device of claim 1, wherein thefirst active region is configured to allow the flow of carrierstherethrough at a first energy level, the second active layer isconfigured to allow the flow of carriers therethrough at a second energylevel, and wherein a change in the charging state of the first quantumdot causes a change in the relative energies of the first and secondenergy levels.
 3. The optical device of claim 2, wherein said first andsecond energy levels can be aligned to allow resonant tunnelling throughthe barrier region.
 4. The optical device of claims 3, comprising anemitter and a collector located on either side of said barrier region,the detector being configured such that carriers flow from the emitterto the collector when the first and second energy levels are aligned. 5.The optical device of claim 1, wherein the means for changing thecharging state of said first quantum dot are provided by the collectorand the emitter.
 6. The optical device of claim 1, wherein the means forchanging the charging state of the first quantum dot comprises means forseparating a photo-excited electron-hole pair and for sweeping at leastone of said electron or hole into the first quantum dot.
 7. The opticaldevice of claim 6, further comprising an absorption region, whereinradiation absorbed by the absorption region excites an electron-holepair in the absorption region.
 8. The optical device of claim 1, whereinthe first quantum dot is configured such that incident radiation excitesa transition within said first quantum dot, and said means for changingthe charging state of said first quantum dot is configured to sweep aphoto-excited carrier out of said first quantum dot.
 9. The opticaldevice of claim 8, wherein the first quantum dot is configured such thatincident radiation excites an electron-hole pair within the firstquantum dot.
 10. The optical device of claim 8, wherein the firstquantum dot is configured such that incident radiation causes anintraband transition within said first quantum dot.
 11. The opticaldevice of claim 1, wherein the first active region is configured tosupport a two dimensional confinement of the carriers.
 12. The opticaldevice of claim 2, wherein the first active region comprises a quantumdot configured to define the first energy level.
 13. The optical deviceof claim 12, wherein the quantum dot configured to define the firstenergy level is a first quantum dot.
 14. The optical device of claim 1,wherein the first active region comprises a bulk three dimensionalsystem.
 15. The optical device of claim 1, wherein the second activeregion is a quantum well configured to support a two dimensional carriergas.
 16. The optical device of claim 2, wherein the second active regioncomprises a second quantum dot configured to define the second energylevel.
 17. The optical device of claim 1, further comprising a secondbarrier region, said second barrier region being located on an opposingside of the second active region with respect to the first barrierregion, the second barrier region being configured such that alignmentof the first and second energy levels allows tunnelling of carriersthrough of the second barrier region in addition to the first barrierregion.
 18. The optical device of claim 1, further comprising aplurality of layers, each layer comprising a plurality of first quantumdots.
 19. The optical device 18, wherein the first quantum dots are ofdifferent sizes.
 20. The optical device of claim 1, wherein the opticaldevice comprises at most 1000 first quantum dots.
 21. The optical deviceof claim 7, wherein the absorption region comprises Iridium.
 22. Theoptical device of claim 7, further comprising a substrate which has asubstantially different lattice constant to that of the absorptionregion.
 23. The optical device according to claim 7, further comprisingstrain reducing means for reducing the strain in an active region due toa lattice mismatch between an active region and the substrate.
 24. Aphoton detector array comprising: a plurality of optical elements, eachoptical element comprising an optical device including, a barrier regionprovided between first and second active regions, and configured toallow tunnelling of carriers from the first region to the second regionthrough the barrier region, at least one first quantum dot configuredsuch that a change in the charging state of the first quantum dot causesa change in the conditions for the tunneling of carriers through thebarrier region, and means for changing the charging state of said firstquantum dot in response to irradiation.
 25. The photon detector array ofclaim 24, further comprising a grid of bit-lines and word-lines, whereinan element is addressable by applying an appropriate bias to a word-lineand/or bit-line, and wherein one of said word lines or said bit lines isconfigured to apply a collector bias and the other of said word lines orbit lines is configured to apply an emitter bias.
 26. The photondetector of claim 24, wherein each optical element comprises a pluralityof first quantum dots, and at least one first quantum dot in at leastone of the optical elements is configured to absorb radiation at adifferent wavelength than at least one other first quantum dot in thesame optical element.
 27. The memory structure of claim 1, wherein thefirst quantum dots of each optical element are configured to maintaintheir charging state even when the power to the bit lines and word lineis switched off.
 28. The memory structure of claim 1, wherein at leastone optical element comprises a plurality of first quantum dots, andwherein at least one first quantum dot in the at least one opticalelement is configured to absorb radiation with a different wavelength tothat of at least one other first quantum dot within the same opticalelement.
 29. An optical device comprising: a barrier region providedbetween first and second active regions, and configured to allowtunnelling of carriers from the first region to the second regionthrough the barrier region; and optically activated means for varyingconditions for the tunnelling of carriers through the barrier region inresponse to irradiation.
 30. A method of fabricating an optical devicecomprising: forming a barrier region between first and second activeregions, the barrier region configured to allow tunnelling of carriersfrom the first region to the second region through the barrier region;forming at least one first quantum dot as part of a layer comprising aplurality of quantum dots, the first quantum dot configured such that achange in the charging state of the first quantum dot causes a changethe conditions for the tunnelling of carriers through the barrierregion; and forming means for changing the charging state of said firstquantum dot in response to irradiation; forming at most 10 monolayers ofa quantum dot layer overlying and in contact with a first layer having asubstantially different lattice constant to that of the quantum dotlayer, such that the quantum dot layer forms islands; and forming asecond layer overlying said quantum dot layer and having a substantiallydifferent lattice constant to that of the quantum dot layer, such that aplurality of encapsulated quantum dots are formed.
 31. An optical devicecomprising: a barrier region provided between first and second activeregions and configured to allow tunneling of carriers from the firstregion to the second region through the barrier region; at least onequantum dot configured such that a change in the charging state of thequantum dot causes a change in the conditions for the tunneling ofcarriers through the barrier region; an absorption regions configured toabsorb radiation and to thereby excite an electron-hole pair; and meansfor changing the charging state of said quantum dot in response toirradiation, the means for changing the charging state including meansfor separating a photo-excited electron-hole pair, from the absorptionregion, and means for sweeping at least one of an electron or hole ofthe separated electron-hole pair into the quantum dot.
 32. An opticaldevice comprising: a barrier region provided between first and secondactive regions and configured to allow tunneling of carriers from thefirst region to the second region through the barrier region; at leastone quantum dot configured such that a change in the charging state ofthe quantum dot causes a change in the conditions for tunneling throughthe barrier region; and means for changing the charging state of saidquantum dot in response to irradiation, wherein the quantum dot isconfigured such that incident radiation excites a transition within saidquantum dot, said means for changing the charging state of said quantumdot is configured to sweep a photo-excited carrier out of said quantumdot.
 33. An optical device comprising: a barrier region provided betweenfirst and second active regions and configured to allow tunnelling ofcarriers from the first region to the second region; at least onequantum dot configured such that a change in the charging state of thequantum dot causes a change in conditions for the tunnelling of carriersthrough the barrier region, the charging state of the quantum dotchanging in response to irradiation.
 34. The optical device of claim 33,further comprising an absorption region configured to absorb theirradiation and thereby photo-excite an electron-hole pair, wherein thecharging state of the quantum dot changes in response to separation ofthe photo-excited electron-hole pair.
 35. The optical device of claim34, wherein the charging state of the quantum dot changes in response toeither of an electron or hole of the separated electron-hole pair beingswept into the quantum dot.
 36. The optical device of claim 33, whereinthe charging state of the quantum dot changes in response to absorptionof a photon by the quantum dot.